1. Field of the Invention
The present invention relates to a printing press that uses an ink transfer roller disposed between an ink fountain and an ink roller to transfer ink stored in the ink fountain to the ink roller and a method of controlling the ink transfer roller.
2. Related Art
In conventional printing presses (e.g., sheet-fed printing presses), when ink stored in an ink fountain is supplied to a plate cylinder via an ink transfer roller, an ink fountain key provided in the ink fountain is opened to transfer the ink to the ink fountain roller (source roller) and the ink is transferred to the ink transfer roller disposed between the ink fountain roller and an ink roller, thereby transferring the ink from the ink transfer roller to the ink roller. In this arrangement, the ink transfer roller is reciprocated (moved back and forth) once every predetermined number of rotations of the plate cylinder. This arrangement may pose a problem that, for example, when an image having a small image area ratio (i.e., an image having a small image area to which ink is applied) is printed, etc., it is difficult to adjust particularly the amount of ink transfer and thus even if the ink fountain key is set at a minimum gap, an excessive amount of ink may be transferred. In light of this, there was proposed a technique to stop a reciprocating action of the ink transfer roller at a certain timing, as well as adjusting the opening degree of the ink fountain key, such as in Japanese Patent Application Laid-open No. Hei-05-147200.
In an ink supply unit of the above conventional printing press, when the adjustment of the amount of ink to be transferred cannot be achieved merely by adjusting the opening degree of the ink fountain key, the reciprocating action of the ink transfer roller is intermittently stopped. For example, with the arrangement where the ink transfer roller is to be reciprocated only once for two rotations of the plate cylinder, once the reciprocating action of the ink transfer roller has been completed, the ink transfer roller is kept stopped until the next action, which action is made after a predetermined number of rotations of the plate cylinder.
In the above conventional printing press, the ink transfer roller is designed to be simply reciprocated once for a predetermined number of rotations of the plate cylinder and the ink transfer roller is kept stopped thereafter until the plate cylinder has been rotated a predetermined number of times. With this arrangement having the ink transfer roller kept stopped, a rotation ratio between the ink transfer roller and the plate cylinder cannot be set in the middle range between 1/1 (1 to 1) and 1/2 (1 to 2). Accordingly, it is necessary to greatly change the opening degree of the ink fountain key. In addition, since it takes time to reflect the change of the opening degree of the ink fountain key to the density of ink on a print, the relevant operation is difficult to perform and the printing quality is hardly stabilized.
Therefore, it is an object of the present invention to provide a printing press and a method of controlling an ink transfer roller that are able to stabilize printing quality.